One of the challenges in building large data centers is that the cost of providing the same communication bandwidth between an arbitrary pair of servers grows in proportion to the size of the cluster. In top-of-rack (ToR) architecture used for data centers, the ToR switch that connects a group of servers in a given server rack to other ToR switches and servers may need to exchange data with other ToR switches. However, increasing the utilization of ToRs while preventing debilitating network congestion can be difficult. Since ToRs typically exchange large data volumes with only a few other ToRs in the data center at any given point in time, the sparse nature of the demand matrix may translate to substantial bottlenecks under a conventional data center topology. In other words, as a handful of ToRs lag behind, they can hold back the entire network from completing its tasks.
This implies that it can be costly to scale data centers to accommodate emerging distributed computing applications. As the number of servers, server racks, and other pieces of equipment increases, it becomes more costly and unmanageable to run the data center at a similar performance level.